The present invention relates to a photoelasticimeter and more particularly to a two wavelength photoelasticimeter which permits the unambiguous determination of the phase shifts which occur in a polarized light wave when the light wave is transmitted through a model.
Photoelasticimetry is the science which is directed to determining the direction of and the difference between the principal stresses of a transparent model subjected to static or dynamic stresses by determining the changes made in the polarization of an incident light wave. During the last few years, optical methods of measurement have been developed which have lead to point determination of the polarization parameters (or forms of light). In particular, various apparatus have been developed which operate automatically and employ as one of their essential components a polarizer rotating at constant speed.
Reference may be had, for instance, to U.S. Pat. No. 3,580,681 granted May 25, 1971. The apparatus disclosed in this patent makes it possible to achieve a precision of one-tenth of a degree in the measurement of the phase variations of the polarized wave. However, the phase measurements are obtained in the form of their sine function and therefore, in the general case, there is an indeterminance caused by multiples of the constant angular values, that is, there is an indeterminance of k.pi.. Consequently, these apparatus lend themselves to entirely automatic measurements only when the maximum variations of the phase shift are less than .pi. radians which occur, for example, when the models are formed from poorly photoelastic transparent materials such as plexiglass or else materials under moderate stress. Under higher stresses leading to phase shifts greater than .pi. radian, it is not possible to measure the phase shift at a point in an entirely automatic manner. Rather, it is necessary to add to the automatic measurement a manual measurement in order to do away with the indeterminance.
One object of the present invention is to provide a completely automatic photoelasticimeter which makes it possible to do away with the k.pi.-radian uncertainty of the prior art.
Another object of the present invention is to provide such an apparatus which is simple to manufacture and easy to use.